A gas turbine engine generally includes, in serial flow order, a compressor section, a combustion section, a turbine section and an exhaust section. In operation, air enters an inlet of the compressor section where one or more axial or centrifugal compressors progressively compress the air until it reaches the combustion section. Fuel is mixed with the compressed air and burned within the combustion section to provide combustion gases. The combustion gases are routed from the combustion section through a hot gas path defined within the turbine section and then exhausted from the turbine section via the exhaust section.
In particular configurations, the turbine section includes, in serial flow order, a high pressure (HP) turbine and a low pressure (LP) turbine. The HP turbine and the LP turbine each include various rotatable turbine components such as turbine rotor blades, rotor disks and retainers, and various stationary turbine components such as stator vanes or nozzles, turbine shrouds, and engine frames. The rotatable and stationary turbine components at least partially define the hot gas path through the turbine section. As the combustion gases flow through the hot gas path, energy is transferred from the combustion gases to the rotatable and stationary turbine components.
Typical turbine nozzles, such as high pressure and low pressure turbine nozzles, have fixed vane configurations and fixed nozzle throat areas therebetween in view of the severe temperature and high pressure loading environment in which they operate. The throat areas between adjacent nozzle vanes must be accurately maintained for maximizing performance of the engine, yet the hot thermal environment requires that the turbine nozzle be manufactured in circumferential segments for reducing thermal stress during operation. The nozzle segments therefore require suitable inter-segment sealing to reduce undesirable flow leakage, which further complicates turbine nozzle design.
Variable cycle engines are being developed for maximizing performance and efficiency over subsonic and supersonic flight conditions. Although it would be desirable to obtain variable flow through turbine nozzles by adjusting the throat areas thereof, previous attempts have proved impractical in view of the severe operating environment of the nozzles. For example, it is common to provide variability in compressor stator vanes by mounting each vane on a radial spindle and collectively rotating each row of compressor vanes using an annular unison ring attached to corresponding lever arms joined to each of the spindles. In this way, the entire compressor vane rotates or pivots about a radial axis, with suitable hub and tip clearances being required for permitting the vanes to pivot.
Applying the variable compressor configuration to a turbine nozzle has substantial disadvantages both in mechanical implementation as well as in aerodynamic performance. The severe temperature environment of the turbine nozzles being bathed in hot combustion gases from the combustor typically requires suitable cooling of the individual vanes, with corresponding large differential temperature gradients through the various components. A pivotable nozzle vane increases the difficulty of design, and also results in hub and tip gaps which require suitable sealing since any leakage of the combustion gas therethrough adversely affects engine performance and efficiency which negates the effectiveness of the variability being introduced.
Furthermore, nozzle vanes are subject to substantial aerodynamic loads from the combustion gas during operation, and in view of the airfoil configuration of the vanes, a substantial load imbalance results from the center-of-rotation of the individual vanes being offset from the aerodynamic center-of-pressure. This imbalance drives the required actuation torque loads upwardly and increases bending loads throughout the nozzle vanes to unacceptable levels.
Such adjustable nozzle vanes necessarily reduce the structural integrity and durability of the nozzle segments in view of the increased degree of freedom therebetween. And, angular pivoting of the individual nozzle vanes directly corresponds with the angular pivoting of the actuating lever arm joined thereto, which renders difficult the implementation of relatively small variations in throat area required for effective variable cycle operation.
Accordingly, it is desired to have a variable area turbine nozzle having improved construction and actuation for improving durability and performance of the gas turbine engine during operation.